Largely due to the hyperactive and impulsive components of attention-deficit/hyperactivity disorder (ADHD), a belief that physical exertion will attenuate disruptive behaviors in children with ADHD has prevailed for some time. Although there is some evidence to support such a claim, other research linking exercise to improved behavior is not compelling. However, methodological limitations with respect to the exercise regimens and the procedures used to assess behavioral change in these studies make it difficult to confirm the influence of exercise on deficit behaviors in children with ADHD. Thus, the purpose of this preliminary study is to examine the effect of a single bout of high-intensity, intermittent exercise, performed at a constant work rate and duration, on attention and impulsivity in children with ADHD. The subjects will consist of pre-pubertal boys (n = 24) and girls (n = 24) 8-11 years of age with ADHD and pre-pubertal boys (n = 12) and girls (n = 12) of a similar age without ADHD. The subjects will be divided into three groups. One group will comprise 12 boys and 12 girls with ADHD. These children will be studied while on their usual dose of medication. A second group of 12 boys and 12 girls with ADHD will be studied 12-18 hours after their last dose of medication from the previous day. The children without ADHD will comprise the control group. Preliminary testing will include a practice session to familiarize the subjects with the psychometric assessment and exercise testing procedures, and a graded exercise test to determine maximal aerobic work rate. The experimental condition will involve 20 minutes of intermittent exercise (30 seconds exercise/30 seconds rest) at a work rate equal to 90% of the child's maximal work rate. Based on observations made in animal studies, exercise of this intensity and duration provokes acute alterations in brain neurochemistry that appear to be similar to the effect of stimulant medication. The Conner's Continuous Performance Test (CCPT) will be administered before and immediately after the exercise bouts and will be used as an objective means to assess exercise induced changes in attention and impulsivity in each group of children. The data will be analyzed using a three-way (group by time by gender) ANOVA. Where appropriate, significant findings from the ANOVA will be subjected to post-hoc testing using Bonferroni adjusted t-tests. Correlations will be determined to assess the relationship between ADHD severity and post-exercise CCPT performance. Statistical significance will be set at an alpha level of 0.05.